The invention relates to a charging circuit for a battery such as a lithium ion battery, and particularly to a battery charger in which the constant-current charging region can be prolonged so that the total charging period is shortened.
FIG. 3 is a diagram showing an example of the configuration of a conventional battery charger. Referring to FIG. 3, a prior art battery charger 120 comprises: a control unit 101 which controls a voltage and a current; a voltage detecting unit 102 which compares an output voltage obtained as a result of the voltage division of resistors 105 and 106 with a reference voltage Vr and outputs a control signal; a current detecting unit 103 which detects an output current from a shunt resistor 104 and outputs a control signal; an output switch 107 which turns on or off the output current; and charging terminals 108 to which a battery 109 is to be connected. A charging current is supplied to the battery 109 via the charging terminals 108.
FIG. 4 is a graph showing charging characteristics of a charging circuit of the prior art battery charger. When the battery 109 is connected to the charging terminals 108 and the output switch 107 is turned on, charging is started. In the initial charging period, the control unit 101 controls the charging current Ic' so as to be suppressed to a constant value, in accordance with the feedback of the output signal of the current detecting unit 103. During this period, the charging voltage Vb' of the battery 109 rises from a start voltage Vs as shown in the figure (constant-current charging region). After the charging voltage Vb' reaches (preset voltage V.sub.0 --dropped voltage V') (a voltage equal to a voltage drop caused by the resistances of the output switch 107, the charging terminals 108, the wiring, etc.), the control unit 101 controls the output voltage so as to be constant, in accordance with the feedback of the output signal of the voltage detecting unit 102. Accordingly, the charging current Ic' attenuates as time elapses, and the charging voltage Vb' becomes closer to the preset value V.sub.0 of the constant voltage (constant-voltage charging region). When the control unit 101 detects a current I.sub.0 at a full charge time t.sub.2, the control unit 101 judges that charging is completed, and turns off the output switch 107 so as to terminate charging.
However, charging performed by the conventional charging circuit has the following problems. In a latter half of the charging period, charging is performed at the constant voltage and hence the charging current gradually attenuates. As a result, the charge completing time period t.sub.2 required for attaining the fullcharge of the battery 109 is prolonged. This is inconvenient, and the operation efficiency is degraded. The charge capacity of the battery 109 is represented by (charging current.times.charging time period). As the constant-current charging region in which the maximum charging current is caused to flow is extended, the charge completing time period t.sub.2 is shortened. In the latter half of the charging period, however, charging is performed at the constant voltage V.sub.0 so that the charging voltage Vb' does not exceed the preset voltage. As a result, the charge completing time period t.sub.2 is prolonged.
There is another method in which charging is performed by using a pulse voltage, in order to shorten the charge completing time period t.sub.2. However, this method has a problem in that the open-circuit voltage (OCV) of the battery 109 temporarily exceeds the preset voltage so that the lifetime of the battery 109 is shortened.